Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Serious Beginner (10 points): Complete any level in single player.
Game Master (20 points): Complete the game.
Serious Sam (30 points): Complete the game in single player on serious difficulty.
I am Invincible (10 points): Complete the game in single player without firing a single bullet or shell.
Serious Run (15 points): Complete the game in single player by beating estimated time on each level.
Metropolis King (20 points): Complete Metropolis level in single player on serious difficulty without loading.
Cannon Expert (5 points): Pierce at least 10 enemies with a single cannonball.
Perfect Kill (10 points): Kill all enemies in Karnak in single player.
Look, It's a Secret (30 points): Find at least 50 secrets in single player.
Co-op Beginner (5 points): Complete any level in co-op.
Co-op Hippie (10 points): Complete The Great Pyramid in co-op with blood option set to hippie.
Serious Co-op (35 points): Complete a co-op game on serious difficulty and extra enemy strength on 400%.